by Anna Arutunyan, Special for USA TODAY

by Anna Arutunyan, Special for USA TODAY

ODESSA, Ukraine - Ukrainian forces battled pro-Russian militants Monday in eastern Ukraine as militants in Odessa vowed to retake buildings they were forced out of amid clashes that left more than 40 people dead.

Ukrainian troops pressed forward with an offensive against militants in Slovyansk, a city in eastern Ukraine that has become a center of militant unrest.

Gunfire and multiple explosions could be heard in the city of 125,000 people. The United States and Kiev say the takeovers are an illegal attempt by militants to overthrow the elected government of Ukraine with the help of Moscow.

Interior Minister Arsen Avakov said about 800 pro-Russia forces deployed large-caliber weapons and mortars, Russia's state-controlled Interfax news agency reported. He said four officers were killed and 30 wounded in the fighting.

Pro-Russian Ukrainians in the Black Sea port city of Odessa vowed to take city buildings after attending the funeral of a politician who died in a fire that killed more than 40 people.

Regional parliament member Vyacheslav Markin, an opponent of the Ukrainian government, was buried Monday while about 300 pro-Russian supporters shouted, "Hero, hero!"

Markin died Sunday after he and 41 others were caught in a building fire during clashes Friday between pro- and anti-government factions.

Many people at the funeral wore the St. George black-and-orange ribbon that is a symbol of pro-Russian armed militants who have taken over government buildings.

"This city has never seen such upheaval. Such moments in history are not forgotten," said Pyotr Volkov, a doctor and a former lawmaker who was a friend of Markin's.

Ukrainian Prime Minister Arseniy Yatsenyuk visited Odessa and said Russia is directing "well-prepared and organized action against people, against Ukraine and against Odessa." He blamed government security forces for failing to prevent the fighting in Odessa, calling the deaths a "tragedy of all of Ukraine."

Pro-Russian demonstrators stormed police headquarters Sunday and won the release of 67 people detained there after the weekend fighting.

The fighting in Odessa escalated Friday when supporters of Ukraine's government tried to clear an encampment of Russian sympathizers on Kulikovo Field Square, burning their tents. Pro-Russians took refuge in the trade union hall on the square. Gas bombs thrown by both sides could have triggered the fire.

"I am shocked that this happened, I can't believe it," said Svetlana Lvova, a teacher who came to lay flowers in the burned-out trade union hall. "Husbands and wives are fighting with each other about what side to take."

Flowers, candles and the bloodied clothes of the dead activists mingled with debris, broken glass and ashes. Men sobbed.

"I don't want to be called a separatist. We don't want to secede from Ukraine," said Anatoly Kuznetsov, an archivist in Odessa whose family, like those of many here, has Russian roots. "But we hope that Russia won't leave us in our sorrow. We are weak. We are not aggressive."

Moscow annexed Ukraine's breakaway republic of Crimea and has threatened to invade Ukraine to protect the Russian-speaking population after demonstrations in Kiev toppled the pro-Moscow regime of President Viktor Yanukovych in February amid street clashes that killed about 100 people.

"Maybe we should have spoken out louder about what the people wanted, but we could not be heard," said Volkov on the sidelines of Markin's funeral. "[Markin] spoke out louder than most. Maybe his death will unify and awake people."

Volkov said he supported wider autonomy for Ukraine's regions. He said this should be achieved without Russia's help.

"I think we can decide on our own fate, but for that to happen, the people have to be heard. We have to learn to listen to people with different opinions," Volkov said. "We don't need help from America or from Russia."